Prestressed concrete is a type of reinforced concrete that has been subjected to an external compressive force prior to the application of loads. The compressive force is typically provided by steel tendons within the concrete that are initially tensioned with hydraulic jacks and held in tension by end anchors.
Prestressed concrete is categorized as either pre-tension or post-tension. Pre-tension refers to the method of first stressing tendons and then casting concrete around the prestressed tendons. The concrete cures before releasing the prestressed tendons and transferring the stress from the tendons to the concrete. Post-tension refers to the method of casting concrete around unstressed tendons and then stressing the tendons after the concrete has reached a specified strength.
Although the modern method of prestressing concrete may be traced to the late 1920's, its general use in the United States did not begin until the late 1940's or early 1950's. General acceptance and the primary increase in use occurred primarily between 1965 and 1975. Application of prestressing was being made in all aspects of construction including buildings, towers, floating terminals, ocean structures and ships, storage tanks, nuclear containment vessels, bridge piers, bridge decks, foundations, soil anchors, and virtually all other types of installations where normal reinforced concrete was acceptable. Thus, prestressed concrete and methods for its initial installation for diverse applications is now well known.
After years of service, however, problems associated with prestressed concrete structures became apparent. The problems primarily related to the premature deterioration of prestressed concrete structures and the subsequent task of identifying and repairing the deteriorated structures before they experienced complete loss of structural integrity.
It is now known that the chloride ion is responsible for the premature deterioration/corrosion of reinforcement steel within concrete. The chloride ion, typically in the form of calcium chloride or sodium chloride, is commonly used on roadways and bridges as a de-icer. Automobiles carry chloride ions to concrete structures such as parking ramps (ie. garages). Once in contact with a concrete structure, the chloride ions leach or otherwise move into the concrete and react with the reinforcement steel within the concrete causing corrosion/deterioration. Due to the corrosive effects of the chloride ion, bridges and parking structures estimated to have useful maintenance free lives of 10 to 25 years are now requiring maintenance in only 7 to 15 years.
The corrosion of prestressed steel is a serious problem that can compromise the structural integrity of a bridge or parking structure with minimal visual signs. Consequently, the early detection of steel failure within a structure is a major safety issue for parking structure and bridge owners. Without detection, steel corrosion can occur to the point of collapse without any major outward visual signs.
Methods of locating steel failures can be categorized as either destructive or nondestructive. Destructive methods involve the removal, often via a jack hammer, of concrete to allow the steel to be physically/visually inspected. Consequently, destructive test methods require some degree of repair after the testing has been completed. By contrast, nondestructive test methods do not require repair after the testing has been completed.
There are many nondestructive tests that are applied to concrete and provide information as to the structural integrity of the concrete. To a limited degree, information provided by such tests can be used to make a semi-educated evaluation regarding the structural integrity of the steel within the concrete. However, to date there is no economical nondestructive method of examining long lengths of prestressing steel with the intent of locating failures. For the most part, existing nondestructive test methods have been limited to the examination of small areas of concrete slab for the sole purpose of locating pretension steel. Examining the entire length of a steel reinforcement with the intent of discovering failures has not been attempted on a systematic large scale production oriented basis.
What is needed is a efficient and reliable nondestructive test method for evaluating the structural integrity of an entire structure such as a bridge or parking ramp. What is also needed is a nondestructive test method for charting entire lengths of reinforcing steel within a structure to identify steel failures. Also what is needed is a method for providing a graphical representation of all of the prestressing steel within a given structure.